/Engine/Source/Runtime/Engine/Classes/Camera/CameraModifier.h
Called per tick that the modifier is active to allow Blueprinted modifiers to modify the camera's postprocess effects. Scaling by Alpha happens after this in code, so no need to deal with that in the blueprint.
void BlueprintModifyPostProcess ( float DeltaTime, float & PostProcessBlendWeight, [FPostProcessSettings](API\Runtime\Engine\Engine\FPostProcessSettings) & PostProcessSettings )